Formaldehyde is a widely used toxic industrial chemical; the present exposure allowed by the Occupational Safety and Health Administration is 3 ppm TLV for eight hours. Carbon monoxide is also commonly present in industrial environments, so it is necessary in practice for a determination of formaldehyde to be independent of interference from carbon monoxide.
Electrochemical gas sensors and devices are well known in the prior art. See, e.g., U.S. Pat. Nos. 4,184,937; 3,992,267; 3,824,167; and 3,776,832. These sensors and devices are used to detect a variety of noxious gases, including hydrogen sulfide, chlorine, nitric oxide, carbon monoxide and various hydrocarbons.
The electrochemical oxidation of formaldehyde to carbon monoxide has been demonstrated on a platinum electrode [Bagotsky et al., Electrochemica Acta 9, 869 (1964) and Kutschkev, ibid 8, 985 (1963)]. However, continuous exposure of a platinum electrode to formaldehyde results in a reduction in the oxidation current with time. This behavior is typical of electrochemical oxidation of organic compounds on noble metal catalysts, which is characterized by rapid fouling of the electrode. The first electron transfer is fast, usually generating an H.sup.+ product and a bound hydrocarbon which does not easily detach from the electrocatalyst. Further, platinum electrodes are also catalytically active to oxidize carbon monoxide, so carbon monoxide interference restricts the use of platinum working electrodes when sensing formaldehyde.